The present invention relates to power conversion apparatus and methods, and more particularly, to pulse width modulator (PWM) power conversion apparatus and methods.
Pulse width modulation is commonly used in switching-type power converters, such as DC power supplies and DC-DC converters. In a typical PWM converter, a control circuit implements power conversion cycles wherein a power source intermittently transfers power to a power transfer network, e.g., an LC filter network, transformer, or the like, for durations that are varied responsive to, for example, an output voltage produced by an output of the power transfer network.
Control of pulse duration in a PWM converter may be accomplished in a number of different ways. Referring to FIG. 1, a control circuit for a conventional PWM converter includes a charging circuit 1 that quickly charges a capacitor 4 responsive to first and second synchronization signals SYNC1, SYNC2. The capacitor 4 is discharged by a current mirror circuit 3 responsive to an output of an amplifier circuit 2. In particular, the amplifier circuit 2 generates a signal that controls the current mirror circuit 3, with the signal representing a difference between and output voltage produced by the PWM circuit and a reference voltage VREF.
The voltage on the charged capacitor 4 is reduced by the current mirror circuit 3 at a rate proportional to the magnitude of the signal produced by the amplifier circuit 2. A voltage responsively produced at an emitter terminal of a transistor 5 is applied to a control input RT/CT of a Unitrode 3843N Current Mode PWM Controller integrated circuit 6. The PWM controller integrated circuit 6 provides a drive signal DRV that has a duty cycle that varies responsive to the voltage applied at the control input RT/CT. This pulse width modulated drive signal DRV may be used, for example, to drive the input switching transistor of a forward converter, or similar switching devices in buck regulator, boost regulator, or other PWM applications.
Other techniques have also been proposed. For example, U.S. Pat. No. 6,130,828 to Rozman describes a DC-DC converter including a transformer having a primary winding connected to an input circuit and a secondary winding connected to an output channel. An amplifier and an integrator circuit are coupled to the output channel and control a switch that is coupled to the secondary winding. A diode coupled between the secondary winding and the integrator circuit resets the integrator circuit. U.S. Pat. No. 6,222,747 to Rinne et al. describes generating a pulse width modulated transistor drive signal responsive to a comparison of a voltage error signal to a sawtooth reference waveform.
According to some embodiments of the invention, a power converter apparatus includes a transformer having a primary winding and a secondary winding, a switching circuit operative to couple and decouple the primary winding of the transformer to a power source, and a control circuit that controls the switching circuit. A feedback circuit is coupled to the output port of a power transfer network and to the control circuit. The feedback circuit generates a current responsive to an output voltage at an output port of a power transfer network, charges a capacitance responsive to the generated current, and intermittently discharges the capacitance responsive to a control signal generated by the control circuit. A modulator circuit is coupled to the feedback circuit and to the power transfer network, and selectively couples the secondary winding of the transformer to the power transfer network responsive to a voltage on the capacitance.
In some embodiments of the invention, the feedback circuit includes an amplifier circuit that generates the current responsive to comparison of the output voltage to a reference. For example, the amplifier circuit may include a transconductance amplifier circuit that generates the current proportional to a difference between a reference voltage and a voltage representative of the output voltage.
In further embodiments, the feedback circuit is further operative to generate a second current responsive to an output current derived from the output port to charge the capacitance responsive to the second current. For example, the feedback circuit may further include an amplifier circuit, e.g., a second transconductance amplifier circuit, that generates the second current responsive to a comparison of the output current to a reference.
According to still further embodiments, the switching circuit includes a first switching circuit, and the modulator circuit includes a comparator circuit that generates a switch control signal responsive to a comparison of the voltage on the capacitance and a threshold voltage and a second switching circuit that selectively couples the secondary winding to the power transfer network responsive to the switch control signal. The apparatus may further include a feedforward circuit that generates the threshold voltage responsive to a voltage applied to the primary winding of the transformer. In yet further embodiments, the switch control signal includes a first switch control signal, and the second switching circuit includes a first switch that operates responsive to a second switch control signal, a second switch that operative responsive to a third switch control signal, and a complementary driving circuit that generates the second and third switch control signals responsive to the first switch control signal such that the first and second switches operate in a substantially complementary fashion. The complementary driving circuit may include a monostable driving circuit that generates the third switch control signal.
The present invention may provide several advantages over conventional PWM converter apparatus. For example, according to some embodiments of the invention, the charging current may monotonically charge a capacitor to generate a comparator input, such that improved noise immunity can be provided in comparison to PWM control circuits that compare a voltage feedback signal to a fixed-slope ramp circuit. In some embodiments, synchronized operation of primary and secondary side modulators can be achieved in a relatively simple fashion by triggering operation of the secondary side modulator using a control signal generated by control circuit that controls the primary side modulator. Such an approach can also provide with additional noise immunity. The invention may be embodied as apparatus or methods.